1. Field of the Invention
The present invention relates generally to an optical transmission module for use in the optical communication field, and more particularly to a receptacle type optical transmission module.
2. Description of the Related Art
In the recent information communications field, high-speed large-capacity processing and high-speed data transmission are required in response to the advancement of information. To meet this requirement, optical transmission is indispensable and preparation is now proceeding toward the expansion and diffusion of an optical communications network.
Known as a device used at many sites in an optical transmission system is an optical transmission module having an optical circuit and an electrical circuit in combination for performing opto-electrical conversion or electro-optical conversion. At present, the production scale of the optical transmission module per communications maker is about 10.sup.5 products per year. However, it is said that the production scale required in the future will become 10.sup.6 or more products per year in response to the diffusion of an optical communications network and that the production cost must be reduced to about 1/10 or less of the present level. Accordingly, it is strongly desired to establish any form of the optical transmission module which can realize mass production and low cost by minimizing the number of components to simplify the assembly process and can also ensure high reliability and long service life.
The components mounted on a printed wiring board built in a communications device are generally classified into a surface mount type and a through hole mount type. A typical example of the surface mount type components is an LSI, which has a form called a flat package. Such a component is soldered to the printed wiring board by a reflow soldering process. This process is performed by printing a solder paste on the printed wiring board, making the surface mount type component stick to the printed solder paste, and heating the whole in a conveyor oven to a solder surface temperature of 220.degree. C. or higher.
A typical example of the through hole mount type components is a large-capacity capacitor or a multi-terminal (200 or more terminals) LSI. The multi-terminal LSI has a terminals form called a PGA (Pin Grid Array). Such a through hole mount type component is soldered to the printed wiring board by a flow soldering process. This process is performed by inserting the terminals of the through hole mount type component into through holes of the printed wiring board, and putting the printed wiring board into a solder bath heated at about 260.degree. C. from the side opposite to its component mounting surface.
In mounting an optical module on the printed wiring board by soldering like the surface mount type component or the through hole mount type component, a so-called pigtail type of optical module with an optical fiber cord is not suitable as the optical module. That is, the optical fiber cord usually has a nylon coating, and the nylon coating has a low resistance to heat at about 80.degree. C., so that it is melted in the soldering step. Furthermore, the optical fiber cord itself invites inconveniences in accommodation and handling at a manufacturing location, causing a remarkable reduction in mounting efficiency to the printed wiring board.
Accordingly, to allow a soldering process for the optical module and reduce a manufacturing cost, the application of a so-called receptacle type of optical module is indispensable. An example of the receptacle type optical module allowing a soldering process is known from 1996 IEICE, General Meeting Proc., C-207 (Ref. 1). In Ref. 1, there is described a receptacle type optical module manufactured by retaining a photoelectric converter and a ferrule with a bare optical fiber on a silicon substrate, next covering the whole with a silicon cap to hermetically seal an optical coupling region, and finally molding the whole with an epoxy resin.
The silicon substrate is formed with a V groove for positioning the optical fiber and the ferrule, both of which are simultaneously fixed by the silicon cap. A lead frame is fixed by an adhesive directly to the silicon substrate, so that the lead frame forms electrical input and output terminals. A commercially available MU type connector housing is mounted on an optical fiber connecting portion to realize connection and disconnection of another optical fiber. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board.
Another example is known from 1997 IEICE, General Meeting Proc., C-361 (Ref. 2). In Ref. 2, a V groove for positioning a bare optical fiber and a ferrule is formed on a silicon substrate as in Ref. 1. The bare optical fiber is fixed to the silicon substrate by a glass plate through a UV curable adhesive, thereby realizing optical coupling between the optical fiber and a photoelectric converter.
An optical coupling region between the photoelectric converter and the optical fiber is sealed by a transparent epoxy resin. The silicon substrate is fixed to a lead frame forming an electrical input terminal, and the lead frame is connected through a gold wire to the photoelectric converter. The whole except an end portion of the ferrule is molded with a resin to form a molded package. An optical connector adapter is mounted onto the molded package to complete an optical module. The optical connector adapter is used to detachably connect another optical fiber to the optical module. By flow soldering of the lead frame extending from the molded package, the optical module is mounted on a printed wiring board.
In an optical subscriber transmission system, economization of the optical transmission system as a whole is also necessary. To this end, there has been proposed and standardized a wavelength division multiplexing bidirectional communication system having a single office terminal to be used commonly by many subscribers. To realize this configuration, an optical module having wavelength multiplexing/demultiplexing functions is required both in each of the subscriber terminals and in the office terminal. In particular, an optical module incorporating a PLC (planar lightwave circuit) formed by integrating the wavelength multiplexing/demultiplexing functions in one chip is expected from the viewpoints of mass production and cost reduction.
In reducing an assembly cost for such a subscriber optical transmission module, it is important to ensure a cost reducing technique for a receptacle structure of an optical fiber interface, especially, an interface between a PLC having wavelength multiplexing/demultiplexing functions and an optical fiber. Conventionally known is a self-alignment technique for the connection between a PLC and an optical fiber. In this conventional technique, a fiber guide is formed on a silicon substrate so as to make alignment of the core of an optical waveguide in the PLC and the core of the optical fiber, thereby determining optimum positions of the PLC and the optical fiber in a self-aligned fashion.
According to such a self-alignment mounting method, it is not necessary to supply a current to an optical semiconductor element, and it is also not necessary to provide a complicated aligning device for aligning the core of the optical waveguide and the core of the optical fiber. Further, no time for the alignment is needed. Accordingly, this method is suitable for mass production and cost reduction.
Known as another example of the receptacle type optical module in the prior art is a technique of optically connecting an optical element and a receptacle ferrule through a V-grooved silicon substrate in a self-aligned fashion. By replacing the optical element with an optical waveguide to follow this prior art technique, it is possible to obtain a structure such that the optical waveguide and the receptacle ferrule are to be optically connected through a V-grooved PLC substrate in a self-aligned fashion.
Also known as another prior art technique is a receptacle type optical module for providing an interface between a PLC having a plurality of optical waveguide cores and multiple optical fibers. In this prior art technique, V grooves for two guide pins are formed on a substrate, and optical coupling between a plurality of optical elements mounted on the substrate or the plurality of optical waveguide cores and the multiple optical fibers is attained through the two guide pins.
The above-mentioned conventional receptacle type optical module has the following problems. First, a deep V groove must be formed on the substrate, so as to mount the ferrule on the substrate. Accordingly, the silicon substrate on which the optical element is mounted or the PLC substrate on which the optical waveguide is formed must be made thick, resulting in an increase in material cost. Further, the substrate must be left under the ferrule, causing a disadvantage in reducing the thickness of the optical module.
Secondly, in the conventional receptacle type optical module, the ferrule mounted in the V groove and the optical element or the optical waveguide core are aligned with each other. Accordingly, there is a possibility of large misalignment between the optical waveguide core (or an active layer in the optical element) and the core of the optical fiber fixed in the ferrule, causing a large optical coupling loss. As a result, characteristics of the optical module are degraded.